1. Field of the Invention
The present invention relates to a cross dichroic prism used for decomposing source light into three primary colors and combining modulated individual color light components which are outputted from light valves in a full-color projector, a method of making the same, and a full-color projector using the same.
2. Related Background Art
Widely used in color-combining and decomposing systems in so-called three-sheet type full-color projectors is a quadrangular dichroic prism in which four isosceles right triangular prisms are assembled together at their rectangular portions, while a dichroic film is formed on thus pasted faces so as to yield an X-shaped cross section.
When this dichroic prism is used in a color-combining system, monochromatic light images of red (R), blue (B), and green (G) are respectively made incident on three faces among four side faces of the quadrangular prism and reflected by or transmitted through the dichroic film disposed at a center thereof, so as to be emitted out of the remaining one side face, thereby forming a combined light image. Here, when adjacent triangular prisms among the four triangular prisms constituting the dichroic prism have refractive indexes totally different from each other, refraction may occur between the adjacent prisms. As a result, emitted light images may not align with each other, thereby generating color shift.
For preventing the color shift, it is necessary to reduce the refractive index difference among the triangular prisms. One of such techniques is disclosed in Japanese Patent Application Laid-Open No. 6-331807.
In this technique, the refractive index difference between at least two sets of triangular prisms adjacent to each other is suppressed to 0.0005 or less. It discloses that, for this purpose, using prisms made in the same lot is important. It also discloses that, as a result, the optical axis deviation among the individual colors, i.e., image deviation, in a quadrangular dichroic prism whose sides each have a length of 40 mm can be suppressed to 13 .mu.m, which is about 1/3 of the pixel size of normally employed light valves, i.e., 40 .mu.m.